286
Exercise 21
be able to place replicate samples under natural aerobic and anaerobic conditions and to be
able to sacrifice one of the replicate samples at each of several time intervals.
6. Place the litter bags in the littoral areas in appropriate sites. Examples could be:
a. Submersed and floating-leaved plant samples: Place bags (tethered to a wooden stake) on
the surface of the sediments and suspend above the sediments (e.g., 50cm) in well-aerated
water.
b. Emergent plants: Tether bags on the surface of the sediments among particulate detritus
and suspend above the water exposed to the air. The latter site simulates "standing dead" of
many emergent macrophytes that remain aerial long after death. [See, for example, Davis
and van der Valk (1978).]
7. At weekly or biweekly intervals, remove the litter bags from each site. Carefully place the
residual plant detritus into tared aluminium foil trays, dry to constant weight at 105 u C, cool
under desiccation, and weigh. Determine the dry weight and calculate the change in weight
over the time intervals.
8. Ifpossible, remove subsamples of the dry material and grind in an appropriate mill (e.g., Wiley
mill, 40-mesh) or to a fine powder with a clean mortar and pestle. Determine the organic
carbon, nitrogen, and phosphorus content by methods outlined in Exercises 7, 9, and 27.
Determine changes through time.
9. Answer the questions on pp. 287-288.
DECOMPOSITION OF LEAF FALL IN STREAMS
Organic matter that enters aquatic ecosystems from allochthonous sources originates primarily
from terrestrial primary productivity. This organic matter of plant origin occurs in both
dissolved and particulate forms and is transformed variously by microbial degradation and
animal utilization during transport to the receiving lake system. Much of the input of terrestrial
organic matter to streams is in the form of soluble compounds either derived as leachates from
the flora or carried in drainage water from the terrestrial ecosystem in various stages of fungal
and bacterial decomposition.
Particulate organic matter can fall directly into streams from overhanging vegetation, be
transported there by drainage water, or be windblown into the stream. The POM from trees and
ground vegetation can provide highly significant amounts of organic matter to streams, both as
POM and as leached DOM [e.g., Fisher and Likens (1973)]. The inputs can be seasonal,
especially in autumn, to woodland streams passing through deciduous forests. Some of the large
POM may become trapped in the stream channel, as, for example, in organic debris dams or
aggregations ofleaves [see Bilby and Likens (1980)]. Accumulated leaves undergo colonization
by bacteria, fungi, and invertebrates in complex successional patterns within the various
microhabitats [e.g., Suberkropp and Klug, (1976)]. As the resistant plant material is degraded,
solubilized products of decomposition are utilized by bacteria living as stratified populations in
the steep redox gradients of the compacted plant tissue (Suberkropp et aI., 1976). The detrital
POM and its associated microflora serves as a food source for numerous aquatic invertebrates
[cf., review by Cummins (1973)]. The shredding, collecting, and grazing activities of aquatic
insects can accelerate the reduction of the size of PO M and subsequent microbial degradation.
Much of the animal nutrition is obtained from the attached microflora rather than from the
POM itself.
Procedures
1. Obtain a number of dead leaves from two types of trees, for example, "tough" leaves, such as
oak or hickory, and more readily degradable leaves, such as aspen or willow. Conifer leaves
(needles) also could be used as a resistant type (would require large-mesh litter bags as used in
the macrophyte exercise).
Exercise 21
be able to place replicate samples under natural aerobic and anaerobic conditions and to be
able to sacrifice one of the replicate samples at each of several time intervals.
6. Place the litter bags in the littoral areas in appropriate sites. Examples could be:
a. Submersed and floating-leaved plant samples: Place bags (tethered to a wooden stake) on
the surface of the sediments and suspend above the sediments (e.g., 50cm) in well-aerated
water.
b. Emergent plants: Tether bags on the surface of the sediments among particulate detritus
and suspend above the water exposed to the air. The latter site simulates "standing dead" of
many emergent macrophytes that remain aerial long after death. [See, for example, Davis
and van der Valk (1978).]
7. At weekly or biweekly intervals, remove the litter bags from each site. Carefully place the
residual plant detritus into tared aluminium foil trays, dry to constant weight at 105 u C, cool
under desiccation, and weigh. Determine the dry weight and calculate the change in weight
over the time intervals.
8. Ifpossible, remove subsamples of the dry material and grind in an appropriate mill (e.g., Wiley
mill, 40-mesh) or to a fine powder with a clean mortar and pestle. Determine the organic
carbon, nitrogen, and phosphorus content by methods outlined in Exercises 7, 9, and 27.
Determine changes through time.
9. Answer the questions on pp. 287-288.
DECOMPOSITION OF LEAF FALL IN STREAMS
Organic matter that enters aquatic ecosystems from allochthonous sources originates primarily
from terrestrial primary productivity. This organic matter of plant origin occurs in both
dissolved and particulate forms and is transformed variously by microbial degradation and
animal utilization during transport to the receiving lake system. Much of the input of terrestrial
organic matter to streams is in the form of soluble compounds either derived as leachates from
the flora or carried in drainage water from the terrestrial ecosystem in various stages of fungal
and bacterial decomposition.
Particulate organic matter can fall directly into streams from overhanging vegetation, be
transported there by drainage water, or be windblown into the stream. The POM from trees and
ground vegetation can provide highly significant amounts of organic matter to streams, both as
POM and as leached DOM [e.g., Fisher and Likens (1973)]. The inputs can be seasonal,
especially in autumn, to woodland streams passing through deciduous forests. Some of the large
POM may become trapped in the stream channel, as, for example, in organic debris dams or
aggregations ofleaves [see Bilby and Likens (1980)]. Accumulated leaves undergo colonization
by bacteria, fungi, and invertebrates in complex successional patterns within the various
microhabitats [e.g., Suberkropp and Klug, (1976)]. As the resistant plant material is degraded,
solubilized products of decomposition are utilized by bacteria living as stratified populations in
the steep redox gradients of the compacted plant tissue (Suberkropp et aI., 1976). The detrital
POM and its associated microflora serves as a food source for numerous aquatic invertebrates
[cf., review by Cummins (1973)]. The shredding, collecting, and grazing activities of aquatic
insects can accelerate the reduction of the size of PO M and subsequent microbial degradation.
Much of the animal nutrition is obtained from the attached microflora rather than from the
POM itself.
Procedures
1. Obtain a number of dead leaves from two types of trees, for example, "tough" leaves, such as
oak or hickory, and more readily degradable leaves, such as aspen or willow. Conifer leaves
(needles) also could be used as a resistant type (would require large-mesh litter bags as used in
the macrophyte exercise).